Field of the Invention
This invention relates to pattern projectors for automatic focus detection suited to photographic cameras, cine cameras, or video cameras, and more particularly to the improvement of focus detection accuracy in the so-called passive type automatic focus detection with the measurement of a degree of sharpness of an object image formed by the objective lens.
Description of the Related Art
The focus detection systems which are classified into two main groups have been used in photographic cameras, cine cameras, or video cameras.
One group is the passive type as proposed, for example, in Japanese Laid-Open Patent Application No. Sho 54-159259, and another one is the active type as proposed for example, in Japanese Laid-Open Patent Application No. Sho 57-154206.
Of these, the active type is to project a light beam, for example, an infrared light beam, from the camera onto an object to be photographed and receive the reflected light beam from the object while the projected light beam is scanning in synchronism with the light receiving element, when the focus detection is performed, having a merit that even if the object space is dim, or even if the contract of the object is low, the focus detection can be performed with a relatively high accuracy. But, with an object at a long distance from the camera, because the projected light beam does not fully reach, or because the amount of light reflected from the object is not sufficident, there is a demerit that the accuracy of focus detection comes to be low.
The passive type is to perform focus detection be sensing the sharpness of the object image formed on detecting means by the photographic system, having a merit that even when the object distance is very long, focus detection of relatively high accuracy can be performed. However, if the object space is dim, or the contast of the object is low, because the sharpness of the object image is difficult to detect well, there is a drawback that the accuracy of focus detection comes to lower. One method of improving this drawback has been proposed in, for example, Japanese Laid-Open Patent Application No. Sho 49-19810. One of the techniques of this kind is to project a random pattern from the camera onto the object by a light projecting system and sense the reflected pattern image from the object when the focus detection of the photographic lens is performed. However, in the photographic lenses, particularly those in the single lens reflex cameras, there is a wide variety of focal lengths as the photographic lens is interchanged. And, recently, the ordinary use of zoom lenses is becoming prevalent.
Therefore, in the method using the random pattern, both projection and reception of an image of the pattern must be performed through the photographic lens, or by the so-called "full-TTL" method. Or otherwise some of the photographic situations will be found unfavorable. To say, as the focal length of the photographic lens varies, that area of the pattern which is to be measured is caused to vary. For example, when the standard lens is interchanged with a long focal length or telephoto lens, that portion of the pattern which is uniform covers the entire area of the range finder spot. Thus, the accurate detection of the image sharpness is no longer possible to perform easily.
Conversely when the full-TTL method is employed, the above-described drawback does not take place, but instead of it, a light projecting system for projecting the patterns must be mounted within the camera housing. As a result, the size of the camera is very increased, and ghost is produced by the multiple reflection of the projected light beam within the photographic lens mounting. The latter leads to lower the quality of the pattern image when it is projected and received. Thus, it becomes a cause of lowering the accuracy of the focus detection.